Re: [PATCH 10/11] scsi: ufs: Optimize the command queueing code

[Adrian Hunter <adrian.hunter@intel.com>]

On 10/11/2021 02:44, Bart Van Assche wrote:
> Remove the clock scaling lock from ufshcd_queuecommand() since it is a
> performance bottleneck. As requested by Asutosh Das, change the behavior
> of ufshcd_clock_scaling_prepare() from waiting until all pending
> commands have finished into quiescing request queues. Insert a
> rcu_read_lock() / rcu_read_unlock() pair in ufshcd_queuecommand() and also
> in __ufshcd_issue_tm_cmd(). Use synchronize_rcu_expedited() to wait for
> ongoing command and TMF queueing.
> 
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  drivers/scsi/ufs/ufshcd.c | 121 +++++++++++++-------------------------
>  drivers/scsi/ufs/ufshcd.h |   1 +
>  2 files changed, 42 insertions(+), 80 deletions(-)
> 

<SNIP>

> @@ -2698,8 +2653,11 @@ static int ufshcd_queuecommand(struct Scsi_Host *host, struct scsi_cmnd *cmd)
>  
>  	WARN_ONCE(tag < 0, "Invalid tag %d\n", tag);
>  
> -	if (!down_read_trylock(&hba->clk_scaling_lock))
> -		return SCSI_MLQUEUE_HOST_BUSY;
> +	/*
> +	 * Allows ufshcd_clock_scaling_prepare() and also the UFS error handler
> +	 * to wait for prior ufshcd_queuecommand() calls.
> +	 */
> +	rcu_read_lock();

The improvement to flush/drain ufshcd_queuecommand() via RCU should
be a separate patch because it is not dependent on the other changes.

<SNIP>